Evaluation of commonly used tear sampling methods and their relevance in subsequent biochemical analysis

Abstract

The human precorneal tear film is a special body fluid, since it is a complex mixture of proteins, lipids, small bioactive molecules, and their concentrations and relative distribution represent not only the metabolic state of the ocular surface but also the systemic and local homeostasis of the outer eye and the human body. This suggests that biochemical analysis of the precorneal tear film composition may provide a non-invasive tool for diagnosis and monitoring of disease progression or treatment efficacy in human medicine. However, collecting tears is demanding, and obtaining reproducible and unaltered samples is challenging because of the small sample volumes of tears. Several methods are available for tear collection as a preparatory step of precorneal tear film analysis, and the collection method used has to be assessed since it has a critical impact on the effectiveness of the assays and on the quality of the results. Each sampling method has advantages and disadvantages; therefore, it is not easy to choose the appropriate collecting method for tear collection. To overcome these limitations various methods have been recommended by different authors for special aspects of specific tests. The aim of our review was to evaluate tear sampling methods with regard to our ongoing biochemical analysis. *Contributed equally.

An eye on the dog as the scientist's best friend for translational research in ophthalmology: Focus on the ocular surface

“…193,229,235,[245][246][247]249 In particular, the diluting effect of reflex tearing or flush methods may drop the concentration of low-abundant compounds below the analytical limit of quantification, and potentially mask differences between groups due to reduced variance in tear composition. 229,250 A third limitation of microcapillary tubes is the technical difficulty associated with the collection method. In fact, it is nearly impossible (or very challenging) to avoid reflex lacrimation in a consistent manner, even with cooperative patients and experienced personnel 230,231,235,249,251 ; for instance, capillary tear collection by Markoulli et al resulted in tear secretion that was approximately fourfold faster than basal tear flow in humans (4.6 vs. 1.2 µl/min, respectively).…”

“…228,231,234,249 Importantly, the technical challenge of capillary tubes is amplified in animals given their uncooperative nature, and in any patient with aqueous tear deficiency given the low tear volume; tear sampling can be extra slow in these cases, possibly impeded/interrupted if an air bubble or mucinous material enters the capillary lumen. 239 Taken together, although direct tear collection remains the preferred method of some investigators given the "undisturbed" tear sample retrieved, 250 the serious drawbacks listed above have prompted a growing number of clinicians and researchers to consider indirect tear sampling in humans as suitable alternatives. 190,231 It is the authors' opinion that indirect tear sampling is also preferred in dogs, cats and laboratory animals.…”

“…234,236,239,244 Absorbent materials collect tears but can also pick up cellular and extracellular "debris," an attribute considered beneficial by some as the sample obtained is more representative of the dynamic microenvironment at the ocular surface, 268 but also perceived as a limitation by others as the fluid retrieved is not "pure" tears. 250 On this note, the main limitation of indirect sampling is the "invasiveness" of the technique, at risk of promoting reflex tearing and altering the composition of the tear fluid; indeed, several studies showed variable tear composition between directly and indirectly collected samples, with notable differences in the qualitative and quantitative profiles obtained for tear lipids 221 and tear proteins. 216,220,226,269 Another important drawback is related to the adsorptive properties of Schirmer strips or absorbent sponges, that is, incomplete release of tear compounds following extraction 209,214,257,258,265 ; however, the authors believe this limitation can be minimized/controlled with adequate precautions (see Section 3.3).…”

Impact of Flow Rate, Collection Devices, and Extraction Methods on Tear Concentrations Following Oral Administration of Doxycycline in Dogs and Cats

“…Direct tear collection with capillary tubes, although commonly used in clinical research because the sample obtained is readily available for analysis, 8 presents several drawbacks that limit its use for drug quantification. In particular, the volume of tear fluid retrieved is generally low, thus rendering some samples insufficient for analysis 1 unless previous stimulation or topical instillation of saline is performed.…”

Biohazardous Impact of Hydrogen Sulfide Gas Exposure on Tear Film Constituents: Structural Characterization by Ultraviolet Spectroscopy

“…The tear fim lubricates the ocular surface and also nourishes the avascular tissues of the cornea. Since tear film is also an optical refractive medium, its stability is pivotal in achieving appropriate vision (Rentka et al, 2017).This study aims to clarify the ocular bio-hazardous effects induced in tear film due to H 2 S exposure.…”